RE: [linux-lvm] Difference in LVM and LVM2 and their strength/weakness

From: "Alex Chen" <achen packetmotion com>

To: <mauelshagen redhat com>

Cc: LVM general discussion and development <linux-lvm redhat com>

Subject: RE: [linux-lvm] Difference in LVM and LVM2 and their strength/weakness

Date: Mon, 23 Jan 2006 11:23:11 -0800

Heinz,
Thank you so much for the valuable information.
My last questions, I hope, are
1. What commands are need to perform the snapshot and back up?
I assume I need to create a snapshot first, (lvmcreate?)
Mount that snapshot (mount?) and make back up from it (tar or cpio?)
A simple example will be great.
2. Can I do this with Red Hat 9 if I enable LVM1 there?
Thanks
Alex
-----Original Message-----
From: Heinz Mauelshagen [mailto:mauelshagen redhat com]
Sent: Monday, January 23, 2006 1:41 AM
To: Alex Chen
Cc: mauelshagen redhat com; LVM general discussion and development
Subject: Re: [linux-lvm] Difference in LVM and LVM2 and their
strength/weakness
On Sun, Jan 22, 2006 at 06:16:33PM -0800, Alex Chen wrote:
> Heinz, thanks for the information/
>
> The main intention of our interest of LIME to use it for snapshots.
> I am told that it is a very quick way to make backups. Is that true?
Is there any measuring number for this?
Hi Alex,
yes, using logical volume snapshots for backups is one of their main
purposes.
The creation of a snapshot happens instantly (ie. just some allocation
for
snapshot internal store and some lvm metadata updates which is *fast*).
The performance of accessing logical voluems (aka origins) with
snapshots
largely depends on your io patterns to the origin (ie. heavy writes to
the
origin slow access to the snapshot down). See remarks below as well.
>
> >From the recent discussion on snapshots, it seems there are still
some issues, and even a kernel patch.
> Does RHEL 4 contain such patch? Or is there a downloadable patch if
we buy one?
We're working on fixes and they'll show up ASAP.
>
> >From what I've read from the Web regarding LIME is that LVM1 is a
read/only while LVM2 is read/write.
Correct.
> The read/only mode, if I understand correctly, is that the PE where
the data resides is first copied to an exception table, (is this the COW
table?)
> before the destination PE is changed. Is this correct? If so, does
the copy involves actual data movement or only a manipulation of
> of the FAT table?
The exception table holds the information about which chunks (smaller
than PEs)
have been read off the origin and stored in the exception store before a
write
to the origin was allowed.
That happens at the block devive level and therefore below any
filesystem
or arbitrary application on top.
> The LVM2's read/write mode seems to simply mark the PE in the snapshot
but the change is not copied from the changed PE to the snapshot, right?
No, any change to the origin leads to a copy *before* the change
happens.
That's why lots of writes to the origin lead to lots of exceptions and
slow
down access.
If a change to the snapshot is carried out, no copy from the origin is
needed,
just the expcetion table is updated.
>
> In either way, the snapshot seems to be keeping tracks of only the
changes, i.e. delta.
Yes.
> Do we need to first create a full copy of the file system as the base,
or we only need to keep the deltas, if we want to use snapshot for
system backups?
The snapshot covers that for you. No user initated copy or anything
needed,
just create a snapshot, mount (or let your arbitrary application access)
it and run your backup on it.
>
> Sorry of all the questions, some of them may be pretty basic or
ignorant. If you feel I should get the answers from other sources, I
would really apprecaite it if you can give me the pointer.
NP.
Heinz
>
> Alex
>
> ________________________________
>
> From: linux-lvm-bounces redhat com on behalf of Heinz Mauelshagen
> Sent: Sat 1/21/2006 09:08
> To: LVM general discussion and development
> Subject: Re: [linux-lvm] Difference in LVM and LVM2 and their
strength/weakness
>
>
>
> On Fri, Jan 20, 2006 at 05:54:12PM -0800, Alex Chen wrote:
> > Greetings,
>
> Hi.
>
> >
> > I am tasked to investigate the differences between LVM and LVM2.
I am
> > new to LVM so it would be greatly appreciated if someone can explain
or
> > point me to the right direction for the following questions:
> >
> >
> >
> > 1. What does LVM2 have that is not available in LVM?
>
> It has command line extensions beyond lvm (eg, vgs, lvs, pvs
commands).
>
> It's configurable in various regards (eg, device name filters)
>
> The limits of 256 grand total logical volumes and ~64k extents per
logical
> volume are gone.
>
> Logical volume snapshots are asynchronous rather than synchronuous
> and gain better performance.
>
> pvmove (online data relocation) is based on temporary, restartable
mirrors
> which wasn't the case in LVM1.
>
>
> > Or is it a
> > only better architectured implementation of LVM?
>
> I'ld not say 'only', but yes, it is :)
>
>
> > 2. Which version of Red Hat enterprise system uses which LVM?
And
> > does the user need to do any configuration during installation?
>
> RHEL3 comes with LVM, RHEL4 with LVM2.
>
> > 3. If LVM2 is definitely better than LVM, how stable is it?
>
> It is stable with the exception of rare snapshot issues,
> we're fixing right now.
>
> >
> >
> >
> > Thanks.
> >
> > Alex
> >
>
> > _______________________________________________
> > linux-lvm mailing list
> > linux-lvm redhat com
> > https://www.redhat.com/mailman/listinfo/linux-lvm
> > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
> --
>
> Regards,
> Heinz -- The LVM Guy --
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Heinz Mauelshagen Red Hat GmbH
Consulting Development Engineer Am Sonnenhang 11
Cluster and Storage Development 56242 Marienrachdorf
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Mauelshagen RedHat com +49 2626 141200
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